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E D I T O R I A L C O M M E N T

Diagnostic Optimization of Coronary CT Angiography*

William Wijns, MD, PHD,† Shengxian Tu, MSC‡

Aalst, Belgium; and Leiden, the Netherlands

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Combined evaluation of coronary anatomy andmyocardial ischemia has become the holy grail ofnoninvasive diagnostic imaging. Several ap-proaches, such as hybrid imaging combining coro-nary computed tomography angiography (CTA) foranatomy and stress-rest nuclear imaging for perfu-sion, are adding to the complexity, cost, and radi-ation burden of both techniques. In theory, a singleimaging device that could provide both anatomic

See page 1149

and functional assessment would be preferable.Choi et al. (1) describe a post hoc analysis ofcoronary CTA that optimizes and expands itsdiagnostic power such that this single modality canfulfill these requirements.

Essential Findings

Choi et al. (1) evaluated the transluminal attenua-tion gradient (TAG) by 64-slice coronary CTA in370 major coronary arteries and correlated TAGwith coronary stenosis as assessed by coronary CTAand conventional quantitative coronary angiography(QCA). Findings by TAG were also related tocoronary flow velocity evaluated by ThrombolysisIn Myocardial Infarction (TIMI) frame count.

The authors found that the linear regressioncoefficient between TAG and the length from thecoronary ostium decreased consistently and signif-

*Editorials published in JACC: Cardiovascular Imaging reflect the views ofthe authors and do not necessarily represent the views of JACC: Cardio-vascular Imaging or the American College of Cardiology.

From the †Cardiovascular Centre, OLV Hospital, Aalst, Belgium; andthe ‡Division of Image Processing, Department of Radiology, LeidenUniversity Medical Center, Leiden, the Netherlands. Shengxian Tu isemployed by Medis Medical Imaging Systems, Inc., and has a researchappointment at the Leiden University Medical Center. Dr. Wijns has

creported that he has no relationships relevant to the contents of this paperto disclose.

cantly with maximum stenosis severity on a per-essel basis (p � 0.0001) and was related to TIMIrame count (p � 0.0001).

In addition, applying the TAG technique signif-cantly improved the diagnostic accuracy of coro-ary CTA in vessels with calcified lesions andefined the classification of stenosis severity byoronary CTA when using QCA as a referencetandard.

hy This New Approach Is Potentiallymportant and Clinically Relevant

s a promising noninvasive imaging tool, coronaryTA is increasingly used in clinical practice to

valuate coronary artery disease (CAD) and to ruleut obstructive CAD in symptomatic patients withow to intermediate pre-test probability of disease2). Although studies have shown good agreementetween coronary CTA and conventional QCA inhe assessment of coronary stenosis severity (3),isual estimates of stenosis severity are routinelysed in clinical practice.Although the information content of coronary

TA is much superior to the one provided by thetandard 2-dimensional invasive coronary angio-ram, simultaneous 3-dimensional imaging oflaque burden and plaque composition often com-romises our ability to adequately size the degree of

uminal stenosis. Yet current revascularization par-digms are based on the recognition of “significant”uminal diameter reduction. Typically, the presencef �50% diameter stenosis will trigger furtherherapeutic intervention and evaluation of the ap-ropriateness of mechanical revascularization toomplement optimal medical therapy.

As it is now widely recognized, reliable assess-ent of coronary stenosis severity can be extremely

hallenging in patients with complex or calcified

oronary lesions.

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One important finding in the present study (1)was that the diagnostic accuracy of visual interpre-tation of coronary CTA can be significantly im-proved when combined with TAG analysis, espe-cially in patients with calcified lesions.

The second important contribution of the TAGapproach is that it allows the extraction of func-tional data, not just anatomic information, from thecoronary CTA. Recent practice guidelines (4) in-creasingly insist on the need for combined anatomicand functional evaluation of CAD for the optimaluse of treatment modalities, such as coronarystenting and bypass surgery. Yet visual interpre-tation of coronary CTA correlates poorly withfunctional testing. When compared with imagingof stress-induced ischemia or the invasivepressure-derived fractional flow reserve, onlyabout half of the “significant” stenoses by coro-nary CTA were of hemodynamic importance (5).As a consequence, decisions based purely onanatomy may lead to a large proportion of inap-propriate revascularization procedures. In thepresent study, TAG analysis correlated with cor-onary flow velocity evaluated by the TIMI framecount, showing at least the potential of coronaryCTA to provide an integrated assessment of thehemodynamic impact of coronary stenosis as wellas its anatomic severity.

Thus, applying both features of TAG analysisto coronary CTA interpretation may represent auseful approach to correct one of the majorlimitations of coronary CTA (i.e., its significantoverestimation of both anatomic and functionalcoronary severity).

Unanswered Questions

The current approach deserves to be further refinedfrom at least 4 aspects.

1. From a methodological viewpoint, it remains tobe determined to what extent beam hardeningand partial volume effect may affect TAG anal-ysis. Perhaps even more importantly, the 64-slice coronary CTA scanner used in the presentstudy does not image the entire coronary tree atthe same time. Therefore, the impact of morerecent scanning protocols with 320-detectorrow machines needs to be evaluated and mayactually increase the incremental diagnosticvalue of the TAG approach.

2. TAG was derived by the linear regression co-efficient calculated over the entire vessel from

proximal to distal at 5-mm intervals. Although a

intervals comprising significant stenosis, se-vere calcification, and stent were excluded toreduce bias caused by the nonlinearity ofluminal attenuation, flow can also signifi-cantly diverge at coronary bifurcations. Thiswill cause nonlinearity of luminal attenuationacross the bifurcations. Therefore, future it-erations of the analysis protocol might benefitfrom including dedicated bifurcation analysisapproaches (6).

3. This study showed that the classification ofcoronary CTA stenosis severity by visual esti-mation was refined with the combined use ofTAG analysis. However, it might be interestingto explore the incremental diagnostic improve-ment per stenosis severity classes, especially formild to intermediate stenoses with uncertainsignificance. A refined diagnosis will be mostclinically relevant when it results in stenosisreclassification from mild class (30% to 49%diameter stenosis) to the moderate class (50% to69% diameter stenosis) or the reverse.

4. A refined TAG technique deserves to be furthervalidated against more sophisticated standardsthan presently used.

From the anatomic perspective, to fully under-stand the added value of the technology, referencestenosis severity by coronary CTA should be ana-lyzed quantitatively using dedicated 3-dimensionalsoftware instead of 2-dimensional technology. Theestimation of the reference diameter, in particular,will be more accurate with the use of 3-dimensionalQCA by integrating information from 2 projec-tions (7).

From the functional viewpoint, the lesion-specific fractional flow reserve is the obvious stan-dard of reference with which TAG estimates ofhemodynamic stenosis severity should be comparedin future prospective studies (5).

Pending further refinement and validation,TAG analysis might help coronary CTA tobecome one of the serious contenders for themost wanted position of “one-stop shop coronaryimaging” tool.

Reprint requests and correspondence: Dr. William Wijns,ardiovascular Centre, OLV Hospital, Moorselbaan64, 9300 Aalst, Belgium. E-mail: William.Wijns@olvz-

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R E F E R E N C E S

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. Arbab-Zadeh A, Hoe J. Quantificationof coronary arterial stenoses by multide-tector CT angiography in comparisonwith conventional angiography methods,caveats, and implications. J Am Coll Car-diol Img 2011;4:191–202.

. Boogers MJ, Schuijf JD, Kitslaar PH,

et al. Automated quantification of ste-

nosis severity on 64-slice CT: a com-parison with quantitative coronary an-giography. J Am Coll Cardiol Img2010;3:699–709.

. Wijns W, Kolh P, Danchin N, et al.Guidelines on myocardial revascular-ization: the Task Force on Myocar-dial Revascularization of the Euro-pean Society of Cardiology (ESC)and the European Association forCardio-Thoracic Surgery (EACTS).Eur Heart J 2010;31:2501–55.

. Sarno G, Decraemer I, VanhoenackerPK, et al. On the inappropriateness ofnoninvasive multidetector computed

tomography coronary angiography to

trigger coronary revascularization: acomparison with invasive angiogra-phy. J Am Coll Cardiol Intv 2009;2:550 –7.

. Tuinenburg JC, Koning G, Rares A,Janssen JP, Lansky AJ, Reiber JH.Dedicated bifurcation analysis: basicprinciples. Int J Cardiovasc Imaging2011;27:167–74.

. Tu S, Holm NR, Koning G, MaengM, Reiber JH. The impact of acquisi-tion angle difference on three-dimensionalquantitative coronary angiography.Catheter Cardiovasc Interv 2011;78:

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